Ultimately, it was more convenient for him to use the seer stone

Joseph used his white seer stone sometimes "for convenience" during the translation of the 116 pages with Martin Harris; later witnesses reported him using his brown seer stone.[1]

The Nephite interpreters which were given to Joseph along with the plates consisted of two stones set in a bow, resembling a pair of "large spectacles." Martin Harris described the Nephite interpreters as being "about two inches in diameter, perfectly round, and about five-eighths of an inch thick at the centre.... They were joined by a round bar of diver, about three-eights of an inch in diameter, and about four inches long, which with the two stones, would make eight inches."[2]

The Nephite interpreters, therefore, were yet another set of seer stones. It is unsurprising that Joseph would be completely comfortable with these instruments, given his experience with the use of seer stones up to that time.

Latter-day Saints associate the term "Urim and Thummim" with these interpreters. Gardner notes,

We all know that Joseph used the Urim and Thummim to translate the Book of Mormon—except he didn't. The Book of Mormon mentions interpreters, but not the Urim and Thummim. It was the Book of Mormon interpreters which were given to Joseph with the plates. When Moroni took back the interpreters after the loss of the 116 manuscript pages, Joseph completed the translation with one of his seer stones. Until after the translation of the Book of Mormon, the Urim and Thummim belonged to the Bible and the Bible only. [51] The Urim and Thummim became part of the story when it was presented within and to the Great Tradition. Eventually, even Joseph Smith used Urim and Thummim indiscriminately as labels generically representing either the Book of Mormon interpreters or the seer stone used during translation. [52] [3]

After the loss of the 116 pages, contemporary accounts are very clear that Joseph continued the translation using his seer stone. In later years, the term "Urim and Thummim" was retroactively applied to both the Nephite interpreters and to Joseph's seer stone. Thus the use of "Urim and Thummim" tends to obscure the fact that two different instruments were employed.

↑J. V. Coombs, Religious Delusions: Studies of the False Faiths of To-Day as cited in Gardner, Joseph the Seer. Van Wagoner and Walker note that "These stones could not have been the Nephite interpreters, yet Joseph specifically calls them 'Urim and Thummim.' The most obvious explanation for such wording is that he used the term generically to include any device with the potential for 'communicating light perfectly, and intelligence perfectly, through a principle that God has ordained for that purpose,' as John Taylor would later put it." as cited in Gardner, Joseph the Seer.